The ratio of prompt negative muons to negative pions produced in the forward direction by the interaction of 400-GeV protons on copper has been determined to be 5.8 × ${10}^{-5\mathrm{}}$ for 55-GeV muons, 3.05 × ${10}^{-5\mathrm{}}$ for 102-GeV muons, and 1.93 × ${10}^{-5\mathrm{}}$ for 230-GeV muons. The ratio of prompt positive muons to negative muons was about 1.

• Received 6 October 1975

DOI:https://doi.org/10.1103/PhysRevLett.35.1613